Conformal coating is the process of applying a dielectric material onto an electrical component, for example, a printed circuit (“PC”) board or a device mounted thereon, to protect it from moisture, fungus, dust, corrosion, abrasion and other environmental stresses. Common conformal coating materials include silicone, acrylic, polyurethane, epoxy synthetic resins and various polymers. When applied to PC boards, an insulative resin film of uniform thickness is formed as a solvent evaporates or, as a solvent free material is cured. A conformal coating can be applied using a dipping process or an automated applicator. However, more current applications require that the conformal coating be applied to selected areas of the PC board and the components thereon in order to preserve electrical and/or heat conduction properties on specific uncoated areas. Further, different conformal coating materials may be required; and it is often desirable to apply the conformal coating materials in different patterns, for example, a straight bead, a bead that is continuously rotated in a curved or circular pattern, and/or a bead that is atomized.
Automated systems are known, which have one or several conformal coating dispensers mounted on a robotic system, wherein one dispenser applies a bead of the coating material; and another dispenser sprays the coating material. Thus, while the multiple dispensers are effective to selectively apply different conformal coating patterns, such a system has several disadvantages. The use of multiple dispensers increases the cost of, the payload on, and the size of, the robotic system, as well as the probability of equipment failure, thereby reducing the reliability of the system. Further, the use of multiple dispensers increases the processing cycle time by the time required to switch the coating process between the dispensers. In addition, multiple dispensers increase the wetted parts that must be cleaned and maintained. Therefore, it is desirable to be able to apply different patterns of coating material with a single dispenser.
Automated systems are also known in which a single conformal coating dispenser is mounted on a robotic system and is able to selectively apply a conformal coating material in different patterns, for example, a straight bead, a rotating bead or an atomized bead. However, such dispensers are relatively complex and require a relatively high level of maintenance in order to reliably apply different patterns of conformal coating materials. For example, known dispensers leave a small amount of conformal coating material in a nozzle path downstream of the dispensing valve after the dispensing valve is closed. As this material cures, it can adversely affect subsequent dispensing cycles. Alternatively, it may be necessary to remove the material via a purge cleaning cycle, or it may be necessary to disassemble the nozzle for a thorough cleaning. Any required cleaning reduces the productivity of the dispensing machine and the efficiency of the dispensing process.
For purposes of this document, the term “downstream” refers to a direction or location that follows a normal coating material flow through the applicator, that is, from a liquid inlet of the applicator toward the dispensing orifice. Likewise, “upstream” refers to a direction or location that is opposite a normal coating material flow through the applicator, that is, from the dispensing orifice toward the liquid inlet of the applicator. Also, for purposes of this document, the term “air assist” refers to the use of any gas, such as air or purified nitrogen to affect the shaping of the liquid fluid stream that is sprayed from the applicator.
Conformal coating materials range from solvent based materials that cure by evaporation of the solvent, to “100% solids” conformal coating materials that do not use a solvent and cure via cross-linking mechanism that may utilize exposure to air, moisture, ultraviolet light, etc. “100% solids” refers to the fact that nearly all of the admixture components in the liquid material are present in the cured material and there is no weight loss due to the evaporation of a solvent, although typically small quantities of diluents (less than 1% by weight) may be added that could evaporate or be consumed. With continuing efforts to reduce the amount of solvents that are used in conformal coating materials, the use of 100% solids conformal coating materials is increasing, for the benefit of reducing the consumption of volatile organic compounds (VOCs). Cleaning a dispenser nozzle that is coated with a 100% solids conformal coating material is more challenging because the 100% solids conformal coating material typically cannot be dissolved in a solvent. Further, with known conformal coating dispensers, the nozzle is recessed inside a cap or cylindrical bore at the end of the dispenser. The recessed nature of the nozzle makes cleaning more difficult and often requires that the cap and nozzle be removed for a thorough cleaning. Again, while the nozzle is being cleaned, the dispenser is turned off and out of production; and thus, the time required to clean the nozzle should be minimized. Therefore, there is a need for a conformal coating dispenser that minimizes the need for cleaning the nozzle and permits the cleaning process to be automated.
With known dispensers, during use, a small amount of conformal coating material accumulates on the external surface of the nozzle, which adversely affects the conformal coating process and must be removed by cleaning the nozzle's external surface. Therefore, there is a need to provide a conformal coating dispenser in which such accumulations of conformal coating material on the nozzle are minimized or eliminated.
Known dispensers that apply a conformal coating material in different patterns are limited in the number of conformal coating materials that can be dispensed without difficulty. Conformal coating materials often have material properties that require frequent operator intervention to manually service the applicator, for example, to clean or change the tip. Therefore, such dispensers cannot be applied in fully automatic dispensing cycles across a wide range of applications. Therefore, there is a need for a conformal coating dispenser that is more flexible and has the capability of providing a greater range of conformal coating materials in various dispensing patterns, while minimizing the number of times and length of time required for cleaning.